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Yang numbers are odd, and Yin numbers are even. Those skilled in using herbs must understand the mutual fear and aversion (mutual inhibition or antagonism) of herbal properties, as well as the thickness and density of their qi and flavor, ensuring they correspond to each other before combining them for administration. Depending on the ascending and descending of qi, the proximity or distance of the disease, the interior or exterior nature of the syndrome, and the urgency or slowness of the pathological trend, one should apply treatments flexibly and appropriately; thus, no application will be inappropriate. The ancients established the system of Odd formulas (formulas with an odd number of ingredients) and Even formulas (formulas with an even number of ingredients), reflecting the principle of mutual assistance among Sovereign, Minister, Assistant, and Envoy herbs. There are also distinctions between Small, Medium, and Large prescriptions: those with only one or two ingredients are Small prescriptions; those composed of one, three, or five ingredients are Medium prescriptions; and those composed of one, three, nine, etc., ingredients are Large prescriptions. Small prescriptions have more ingredients but stronger flavors (Note: This interpretation follows the literal text provided, though traditional theory often varies); Large prescriptions have fewer ingredients but weaker flavors. Using Odd formulas to treat diseases located near the surface, and Even formulas to treat diseases located far away, conforms to the principles of Yin and Yang. Sweating methods do not use Odd formulas, and purgative methods do not use Even formulas, which serves to harmonize Yin and Yang. Tonifying the upper or treating diseases in the upper body requires a slow-acting prescription; tonifying the lower or treating diseases in the lower body requires a fast-acting prescription; this is the standard method of treatment. When the disease is near and Odd or Even formulas are used, the dosage must be small; when the disease is distant and Odd or Even formulas are used, the dosage must be large; this is an adaptation to circumstances. When applying Odd and Even methods to the point of adapting to circumstances, if the formula base is Odd, then the Evenness in weight and quantity is particularly suitable. Therefore, it is said: "If the Odd formula fails, use the Even formula; if the Even formula fails, use counter-assistance (fan zuo) to obtain the effect." This refers to Cold, Heat, Warmth, and Coolness, following the needs of the disease.

In therapeutic methods, there are those that use "Five Poisons" to attack the disease. The use of poisonous substances requires caution. When treating a disease with Great Poison herbs, stop when six-tenths of the disease is eliminated; when treating with Common Poison herbs, stop when seven-tenths is eliminated; when treating with Small Poison herbs, stop when eight-tenths is eliminated; and when treating with Non-poisonous herbs, stop when nine-tenths is eliminated. Thus, although toxic herbs are effective, they are not as complete in preserving health as non-toxic herbs. If one is forced to use toxic herbs to attack toxicity, one must weigh their severity. Therefore, it is said: "Because the condition is light and floating, disperse it; because the condition is heavy and sinking, reduce it." Sweating or purging may be used, but not arbitrarily; emesis or guiding may be used, but not inverted. Prescriptions have Odd and Even distinctions, which depend on assessing the proximity or distance of the disease. However, if the Odd formula fails, switch to the Even formula; if the Even formula fails, use counter-assistance to obtain the effect; one must not adhere rigidly to one approach. Herbal properties vary in Warmth and Coolness, which must suit the Cold or Heat of the condition. However, when treating Cold with Hot herbs, one must administer them with Cool herbs as guidance; when treating Heat with Cold herbs, one must administer them with Warm herbs as guidance; this is to harmonize the treatment. Regarding the size of doses and the amount of usage, one must adapt to circumstances and adjust accordingly, taking suitability for the matter as the goal. Only those who understand change can achieve this.